The term “microphone array” is used if two or more microphones, at a distance from each other, are used to receive sound signals (multiple-microphone technique). It is thus possible to achieve directional sensitivity in the digital signal processing. The classic “shift and add” and “filter and add” methods, in which a microphone signal is shifted in time relative to the second one or filtered, before the thus manipulated signals are added, should be mentioned first here. In this way, it is possible to achieve sound extinction (“destructive interference”) for signals which arrive from a specified direction. Since the underlying wave geometry is formally identical to the generation of a directional effect in radio applications when multiple aerials are used, the term “beam forming” is also used, the “beam” of radio waves being replaced by the attenuation direction in the multiple-microphone technique. The term “beam forming” has become accepted as a generic term for microphone array applications, although actually no “beam” is involved in this case. Misleadingly, the term is not only used for the classic two-microphone or multiple-microphone technique described above, but also for more advanced, non-linear array techniques for which the analogy with the aerial technique no longer applies.
In many applications, the classic method fails to achieve the actually desired aim. Attenuating sound signals which arrive from a specified direction is often little use. What is more desirable is, as far as possible, to pass on or further process only the signals from one (or more) specified signal source(s), such as those from a desired speaker.
From EP 1595427 B1, a method of separating sound signals is known. According to the method described there, the angle and width of the “directional cone” for the desired signals (actually not a cone but a hyperboloid of rotation), and the attenuation for undesired signals outside the directional cone, can be controlled by parameters. The described method calculates a signal-dependent filter function, the spectral filter coefficients being calculated using a specified filter function, the argument of which is the angle of incidence of a spectral signal component. The angle of incidence is determined, using trigonometric functions or their inverse functions, from the phase angle between the two microphone signal components; this calculation also takes place with spectral resolution, i.e. separately for each representable frequency. The angle and width of the directional cone, and the maximum attenuation, are parameters of the filter function.
The method disclosed in EP 1595427 B1 has several disadvantages. The results which can be achieved with the method correspond to the desired aim, of separating sound signals of a specified sound source, only in the free field and near field. Additionally, very small tolerance of the components, in particular the microphones, which are used is necessary, since disturbances in the phases of the microphone signals have a negative effect on the effectiveness of the method. The required narrow component tolerances can be at least partly achieved using suitable production technologies, but these are often associated with higher production costs. The near field and free field restrictions are more difficult to circumvent. The term “free field” is used if the sound wave arrives at the microphones 10, 11 without hindrance, i.e. without being reflected, attenuated, or otherwise changed on the signal path 12 from the sound source 13, as shown in FIG. 1a. In the near field, in contrast to the far field, where the sound signal arrives as a plane wave, the curvature of the wave front is shown clearly. Even if this is actually an undesired difference from the geometrical considerations of the method, which are based on plane waves, there is normally great similarity to the free field in one essential point. Because the signal or sound source 13 is so near, the phase disturbances because of reflections or similar are normally small in comparison with the desired signal. FIG. 1b shows the use of the microphones 10, 11 and sound source 13 in an enclosed room 14, such as a motor vehicle interior. However, when used in enclosed rooms, the phase effects are considerable, since the result of the reflections of the sound waves on flat or smooth surfaces in particular, e.g. windscreens or side windows, is that the sound waves are propagated on different sound paths 12, and near the microphones disturb the phase relationship between the signals of the two microphones so greatly that the result of the signal processing according to the method described above is unsatisfactory.
The result of the phase disturbances because of reflections, as shown in FIG. 1b, is that the spectral components of the sound signal of a sound source 13 apparently strike the microphones 10, 11 from different directions. FIG. 2 shows the directions of incidence in the free field (FIG. 2a) and in the case of reflections (FIG. 2b), for comparison. In the free field, all spectral components of the sound signal 15f1, 15f2, . . . , 15fn come from the direction of the sound source (not shown in FIG. 2). According to FIG. 2b, the spectral components of the sound signal 16f1, 16f2, . . . , 16fn, because of the frequency-dependent reflections, strike the microphones 10, 11 at quite different apparent angles of incidence θf1, θf2, . . . , θfn, although the sound signal was generated by one sound source 13. Processing the sound signals in narrower or enclosed rooms, in which only sound signals from a specified angle of incidence are taken into account, gives unsatisfactory results, since in this way certain spectral components of the sound signal are not or inadequately processed, which in particular results in deterioration in the signal quality.
A further disadvantage of the known method is that the angle of incidence as a geometrical angle must first be calculated from the phase angle between the two microphone signal components, using trigonometric functions or their inverse functions. This calculation is resource-intensive, and the trigonometric function arc cosine (arccos), which is required among others, is defined only in the range [−1, 1], so that in addition a corresponding correction function may be necessary.